The interstellar chemistry of C 3 H and C 3 H 2 isomers.

We report the detection of linear and cyclic isomers of C3 H and C3 H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3 Hx with x=1,2) and ionic (C3 Hx + with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3 H2 + and C3 H3 + isomers showing that the statistical treatment of the relaxation of C3 H* and C3 H2 * produced in these DR reactions may explain the relative c,l-C3 H and c,l-C3 H2 abundances. We have also introduced in the model the third isomer of C3 H2 (HCCCH). The observed cyclic-to-linear C3 H2 ratio vary from 110 ± 30 for molecular clouds with a total density around 1×104 molecules.cm-3 to 30 ± 10 for molecular clouds with a total density around 4×105 molecules.cm-3 , a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3 H2 → H + c-C3 H2 and H + t-C3 H2 → H + c-C3 H2 isomerization reactions.